The influence of annealing parameters (temperature and duration) on the crystallization process of vitreous sodium-trimetaphosphate has been studied by X-ray diffraction under air-free conditions. The phenomenon is undoubtly initiated by very small amounts of water being inevitably present. A crystalline phase grows on the sample surface with very strong crystallographic orientations, the structure of which being related to the structure of a sodium-trimetaphosphate hydrate. Water is gradually released when annealing temperature and/or duration is increased. Crystallization of anhydrous trimetaphosphate occurs first on the surface before spreading over the whole volume of the solid.